In the production of motion pictures for theaters and for television, the use of the Blue Screen Image Compositing Process is often used. While image compositing devices have been programmed to use any backing color, Green or Blue are used almost exclusively. Thus, references herein to blue screen should be understood to include any colored backing for which an image compositing device has been programmed. In image compositing the need for the camera to follow the actors often causes the camera frame to overshoot the blue screen. It is common practice to use the window function of the compositing device, such as an Ultimatte 11 available from Ultimatte Corporation, to bring in a window signal (garbage matte) from the edge of the image frame over to the edge of the blue screen. A major function of the blue screen is to generate a control signal, known as a matte or a key signal, that turns on the background scene in all areas of the blue screen not occupied by a subject. The function of a window signal is to turn on the background scene in that area of the image frame not occupied by the blue screen. The window signal is noise free since it is simply a fixed signal level.
A background scene rendered in a computer is also noise free. A matte signal generated by the blue screen contains whatever noise pattern is present in the grain of the camera film, or the electronic noise present in the video camera, which is imprinted over the background scene. Since both the background scene and window (garbage matte) signal are each noise-free, that part of the background turned on by the window remains noise-free. When viewing the background scene in the composite image, one will notice that one area of the background is noisy as compared to the noise free background area controlled by the noise free window signal. Camera noise rarely attracts the attention of a viewer until it is placed next to a noise free area, which makes the noise pattern quite obvious.
One solution to this problem has been to use the cleanup signal to raise the level of the matte signal, derived from the blue screen area, into a dip that removes all camera system noise and uneven screen illumination. Unfortunately, the use of cleanup to eliminate noise also degrades the foreground subject image by removing the thinner strands of hair, reducing shadow density, and eliminating footprints and other marks on a blue floor. In a Virtual Studio, retaining these artifacts is useful and often necessary to provide an impression of reality, which may be lacking when background scenes are rendered on a computer. The opaque portions of the foreground subject, which are not impacted by the dean up matte signal, will retain the noise levels of the camera image. When this foreground subject is composited with a noise-free background scene, the resulting scene will look unnatural, since noise is present only in some parts of the entire image.
The function “Cleanup” is explained in U.S. Pat. No. 4,344,085.